Halve lesson gain when the bag has no textbook for the subject.

Locker and home do not count. A pack without the field keeps vanilla 0.5.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/HSchool.Content/PeopleDefValidator.cs

@@ -506,6 +506,11 @@ internal static class PeopleDefValidator
             throw new ContentLoadException($"BehaviorDef '{behavior.DefName}' lessonSkillPerHour cannot be negative.");
         }
 
+        if (behavior.LessonNoTextbookFactor is < 0f or > 1f)
+        {
+            throw new ContentLoadException($"BehaviorDef '{behavior.DefName}' lessonNoTextbookFactor must be 0–1.");
+        }
+
         if (behavior.SwitchMargin < 0f)
         {
             throw new ContentLoadException($"BehaviorDef '{behavior.DefName}' switchMargin cannot be negative.");